All-aromatic polyetherimides are known to have outstanding (thermo) mechanical properties. However, at the same time, they are also known to be extremely difficult to process. They are either processed from the soluble precursor amic-acid state or via the melt using a reactive oligomer route. High-molecular-weight aromatic thermoplastic polyetherimides (“PEIs”) are rare and their melt viscosities are high, making them difficult to process. A user would prefer an all-aromatic polymeric PEI with liquid crystalline properties. Ideally, the polymer would exhibit liquid crystalline properties in the melt, i.e., show thermotropic behavior. Nematic liquid crystalline polymer phases have the advantage that they show shear-thinning effects, which makes them very easy to process via a variety of melt processing techniques, and complex parts can be made with virtually no mold shrinkage. Additional advantages of liquid crystalline polymers over classic polymers include improved barrier properties towards small molecules (e.g., H2, O2, H2O, etc.).
Several classic PEI formulations, such as Kapton® for example, meet some of the molecular requirements for forming liquid crystal phases. However, the rigid nature of the polymer backbone and/or strong inter-molecular interactions result in highly intractable polymers that never melt; in fact, they will decompose before the melting point is reached (Tm>>Tdec). Liquid crystalline phases, in solution or in the melt, are therefore not observed.
There are liquid crystalline PEIs where aliphatic flexible spacers are used in order to lower the melt transition temperature to acceptable levels. The disadvantage of using such spacers is that they result in polymers with lower mechanical and thermal properties compared to all-aromatic polymers. All-aromatic polymers are often more suitable, for instance, for aerospace and electronic applications.